Archive for the ‘thermocouple’ Category

Photo 1: Hello world, using the WiringPi library to control an LED display.

To celebrate getting the GPIO working using WiringPi from Gordon Henderson I thought I’d have a quick look at the difference between running some code on an Arduino compared to running almost the same code on a Raspberry Pi (RasPi).

It is worth noting now that the setups I’ve been using to test with are all powered from a separate 3.3V supply with the grounds linked. Nothing is being powered via the RasPi itself to avoid drawing too much current should I make a mistake. When working with the RasPi you should be careful as the RasPi will not take kindly to static discharge, connecting things backwards or the wrong voltage levels.

The really great thing about Gordon’s library is that by using it I am able to transfer Arduino code to the RasPi with only a few simple changes. Obviously not everything in the Arduino libraries are present, but the basic I/O manipulation is plenty for some simple applications.

I initially ported the example and library to C as the WiringPi was in C and I had a couple of issues compiling mixed C and C++ properly. After a bit of investigation I found that when explicitly including the libraries as C the compiling problems go away. Below is an example of how to modify the wiringPi.h file to ensure it is included as C and not C++. Gordon has said he will make this change to the library on the next release. Once I had sorted out the makefile to use g++ rather than gcc I was able to compile and link successfully.

While compiling and linking of the example was now okay, the next problem was that when run, the program just exited with a “Segmentation Fault” (segfault). I didn’t even see the error message when running as root with sudo, it was only when running as the standard user (pi) that it became apparent that something was wrong with the software rather than the hardware.

When I had run previous examples as the standard user I would see a permissions related error complaining that the program could not access the “/dev/mem/” device. This error comes from the part of the WiringPi library that sets up memory access to the hardware registers in the Broadcom BCM2835. This clue told me that the wiringPiSetup routine was not being run. This in turn showed me that the global object representing the temperature sensor was being constructed before the WiringPi library was being setup.

The solution to this particular problem was simple enough. I edited the MAX6675 library so that it calls wiringPiSetup directly when initialising. I also set an internal flag if this succeeds. In addition to this I modified the methods to exit with an error if the flag has not been set. This means that none of the MAX6675 library methods should ever call WiringPi functions without the appropriate initialisation having taken place and succeeded beforehand. My code for this example is here. A more general solution might be to add this flag to the WiringPi library itself.

LED Display (MLMC) and Temperature Combined Example

Following my success with the temperature sensor I decided to push the boat out a little further and try to interface to my MLMC LED display system.

The MLMC LED display is a set of LED dot matrix control PCBs I designed to use up some unusual LED modules I had lying about. You can get the full story in this blog post or on the website.

Photo 2: Using the WiringPi library to control an LED display and thermocouple interface

This was a simple demonstration with the RasPi reading from the thermocouple interface and then displaying the temperature on the display. For this example I wrote everything in C. The MLMC interface is very simple and involves clocking 16 bit words of data using two pins, one for data and one for clock. Each word represents a single column on the LED display. This low speed (~5Kbps) synchronous protocol was originally implemented by directly manipulating the digital I/O pins on the Arduino and so was very simple to port to the RasPi with the WiringPi library.

While the ported program functions very well, the waveforms obtained from the RasPi are quite different to those achieved with the Arduino. This is a good example of how the same simple code can be quite different when ported between platforms.

MLMC Clock Waveforms

The function used to send serial data to the MLMC is shown below. This code has a 75µs delay between switching the level of the clock and this gave uniform pulses on both systems. The pulses also appeared consistent from one run to another.

The following scope traces are the result of executing Listing 2 on the Arduino:

Figure 1: Clock pulses from the Arduino to the MLMC modules

Figure 2: Single clock pulse from the Arduino to the MLMC modules

Arduino has a total pulse time of approximately 80µs for the first clock pulse to MLMC. This extra 5µs I assign to the time taken to call the digitalWrite function, a single shift operation and iterating the for-loop.

The next two traces in figure 3 and figure 4 are produced when running the same code on the RasPi:

Figure 3: Clock pulses from the Raspberry Pi to the MLMC modules

Figure 4: Single clock pulse from the Raspberry Pi to the MLMC modules

You can notice that while the pulses look identical at a quick glance the RasPi has 150µs pulse width; almost double that of the Arduino. I assume that the delayMicroseconds function, which only guarantees that a minimum of the given microseconds have elapsed before returning, is releasing control to the operating system and coming back a lot later than expected. As the delay implementation in the WiringPi library simply calls nanosleep I am surprised this operation takes twice as long. There may be a number of causes for this behaviour:

There might be a bug in the way the delay function is implemented.

The shift and for-loop iteration might have compiled in a very inefficient way.

The operating system simply cannot schedule other tasks quickly enough to return from nanosleep and meet our expectations.

MAX6675 thermocouple interface

This loop was clocking data in from the MAX6675 chip via a “bit-banged” interface. The main loop did not have a delay and was toggling bits as quickly as possible while shifting in the value via a single GPIO pin.

The following traces in figure 5 and figure 6 show the results on the Arduino.

Figure 5: Clock pulses from the Arduino to the MAX6675

Figure 6: Single clock pulse from the Arduino to the MAX6675

On the Arduino this code gave a uniform pulse train with the pulse widths measuring approximately 6-7µs. As the Arduino is running at 16Mhz I would like to think this could be optimised a lot more to get something like 120ns if I used some assembly or bypassed the Wiring/Arduino API, but that is an exercise for another blog post. 🙂

Figure 7: Clock pulses from the Raspberry Pi to the MAX6675

Figure 8: Close-up clock pulses from the Raspberry Pi to the MAX6675

The RasPi on the other hand gave a very non-uniform pulse train. As this is synchronous communication, the clock does not need to be uniform, it dictates when the data line is read. The average pulse in this waveform is only 150ns wide which is approximately 48 times faster than the Arduino. Given that the RasPi clock speed of 700MHz is roughly 44 times faster then the Arduino clock speed of 16MHz, this makes sense.

The RasPi would vary any individual pulse length by up to a factor of two (from observation) but did not seem to be effected by increasing the load (multiple ssh sessions and running an openssl speed test). The RasPi would also continue to run the MLMC scrolling display and reading the temperature sensor without issues while another process was also reading the temperature sensor at the same time (single_temp.cpp from first example above). Even while openssl was doing speed testing (cpu usage at 98-99%) the visible operation of the screen was not effected.

Investigating Further

The long delay and changing pulse widths could be investigated further with a couple of short programs.

I could investigate the long delay further by looking at repeating the test with the system under different loads and noting the effect on the width and consistency of the pulses. To vary the workload on the RasPi I could use a tool like stress.

We could also step through many values while keeping the system load constant. This would give us an idea if their was a minimum overhead experienced by the delay function or if their was some sort of error in the calibration.

Conclusions

This was a very quick look into porting code from Arduino to Raspberry Pi. I found that it was not so difficult to port some very simple applications, the majority of the code would simply run unchanged.

While the code may compile and run, the actual operation of the code will always need to be checked carefully for timing issues and other unexpected behaviour when switching hardware and using new library implementations.

As these systems were using synchronous serial communication the exact shape of the waveforms did not actually need to be uniform for the system to work.

On review of the MAX6675 data sheet the timings slighty exceed the stated maximum clock freq of 4.3Mhz. So If I was intending to use this further I would introduce a short delay into the MAX6675 clock routine to ensure it complies with the data sheet.

The MLMC module operates perfectly with the timings achieved but we are operating at half the throughput we expected due to the delay function not operating identically to the same function on the Arduino. While this didn’t cause a problem in this simple test it could easily have done so in a slightly more complicated system.

An Arduino driving five MLMC modules displaying the current temperature

I have finally motivated myself to publish my firmware and PCB designs for the MLMC on github. The design is functional and I have a set of five modules chained together sitting on my workbench. They happily display the current temperature via an Arduino and the MAX6675 based Thermocouple board I designed previously.

Once a couple of initial glitches in the system had been worked out the modules turned out to be both reliable and easy to drive.

The power consumption as on average 12mA per module while displaying scrolling text. The current consumption is reasonably nicely distributed about the average without any large peaks in current. Each module’s refresh rate is slightly out of sync with its neighbour due to the differences in each of the AVRs on-board oscillators. This has the side effect that all columns switch on at a slightly time and avoids causing a large spike in the current. While not deliberately designed to act this way it is a benefit of the multiple controller modular design.

Trying to drive 1280 LEDs from just 4 wires was not without its problems, the three main problems are covered in greater detail in later posts but in brief they were as follows:

Synchronisation issues caused by jitter, noise and missing bits

Clocking out the last word from one module to the next

PCB design error – SPI pins are not always the same as the ISP serial programming pins.

Component Choice. Not enough memory for desired bit depth or bandwidth to achieve original functionality

In an unprecedented move I’ve released another version of the firmware for the AVR Butterfly Logger within a few months of a previous release. This was a minor change just adding MAX6675 based J and K thermocouple support.

You can download the source code from sourceforge, with the changes from the previous version are listed below.

The new library has configurable support for averaging readings but each individual reading takes 200ms so an 8 point average will take 1.6s to complete. If you use this averaging feature please make sure that your logging interval is long enough to ensure enough time for the complete reading to finish. While thermocouples are capable of measuring wide ranges of temperature both positive and negative the MAX6675 is limited to returning readings between 0°C and +1024°C. If the chip detects an open thermocouple then the system will log ‘-1’ as the temperature.

The MAX6675 connects to the AVR Butterfly via the SPI bus. This bus is also used by the onboard Dataflash and the Kionix acceleration sensors (when in use). As this bus is already available the only additional pins required is a single chip select (CS). The library currently only supports a single thermocouple input but the code could easily be expanded to suport multiple MAX6675s requiring a single CS line for each additional chip and thermocouple.

The SPI bus is available on PORTB and also via the ISP connector. An example breakout PCB for the MAX6675 was published here. Default wiring from the this PCB to the AVR Butterfly Logger is given below:

PCB

MAX6675

Butterfly

J403 (ISP)

J405 (USI)

1

1 GND

GND

6

4

2

7 MISO

PB3

1

–

3

6 CS

PE6

–

3

4

5 SCK

PB1

3

–

5

4 VCC

VCC

2

–

More information about the data logger project can be found at the project website.

Using thermocouples with the Reprap Mendel is pretty straight forward, although I did overcome a couple of issues in getting it working.

A known issue with the MAX6675 Arduino/Reprap code

The MAX6675 chip can take up to 220ms to take a reading from the thermocouple and the reading begins automatically after the previous read. If another read is attempted before the current reading has been completed then the chip returns the previous value and restartsthe internal analog to digital the conversion. This means that if you read from the chip every 150ms or so you will always get the same first reading returned again and again.

The taking of a single reading rather than taking a number of readings and averaging will contribute to a high noise level in the thermocouple readings when using this software.

Discovering the problem

When testing the PCB with the library code everything works fine. The arduino returns a reading every second and everything looks good.

When using the Reprap firmware with the thermocouple code it always returned the first value read from the chip (usually 24°C or so). This lead to things getting pretty hot as the extruder would turn the heater to full duty cycle while recording no change in temperature.

I first looked at the SPI lines on the scope to see if there was excessive noise or some other issue. I also compared this side by side with the ‘working’ arduino system. The scope traces are shown below. The reference channels (R1 and R2) are the reprap firmware and the bottom traces (1 and 2) are the arduino. I noticed nothing wrong with the traces, they both looked very good.

It was here however that I noticed that the arduino code calls another read to the chip immediately after the preceding read. After a bit more investigation I noticed that the library software calls the temperature reading function 5 times consecutively and always returns 5 identical results. A quick read of the data sheet confirmed the typical conversion time to be 170ms with a maximum of 220ms. The arduino library starts a conversion just 20μs after finishing reading the last. Reading the data sheet further also confirmed the action of restarting a conversion every time the chip is read.

I monitored the lines on a scope and found that the Reprap software was indeed reading the chip every 136ms. To fix this I simply changed SLOW_CLOCK in configuration.h to 8089 from 5000. Based on the current delay giving 136ms, this gives 220ms delay between calls to the thermocouple chip. After making this change to the firmware the thermocouple now works perfectly.

Time between calls to the MAX6675 thermocouple chip using the standard firmware on my reprap.

Fixing the problem

A more robust fix to the reprap extruder firmware is shown below, but I have not tested this code yet. This simply exits the temperature reading method if enough time has not elapsed between calls to the function. The advantage of this is that the main loop can run as fast as it needs to without impacting on the operation of the thermocouple sensors.

#ifdef MAX6675_THERMOCOUPLE
#define MAX6675_CONVERSION_TIME 220 // number of milliseconds between valid temperature readings
int value = 0;
byte error_tc;
static unsigned long last_read_time=0; // number of milliseconds since power on at last MAX6675 read
unsigned long time_since_last_read=0; // number of milliseconds since last read of MAX6675
/* check the chip is ready to produce a new sample */
time_since_last_read=millis()-last_read_time;
/* only read from the chip if a conversion has been completed */
if(time_since_last_read > MAX6675_CONVERSION_TIME){
digitalWrite(TC_0, 0); // Enable device
/* Cycle the clock for dummy bit 15 */
digitalWrite(SCK,1);
digitalWrite(SCK,0);
/* Read bits 14-3 from MAX6675 for the Temp
Loop for each bit reading the value
*/
for (int i=11; i>=0; i--)
{
digitalWrite(SCK,1); // Set Clock to HIGH
value += digitalRead(SO) << i; // Read data and add it to our variable
digitalWrite(SCK,0); // Set Clock to LOW
}
/* Read the TC Input inp to check for TC Errors */
digitalWrite(SCK,1); // Set Clock to HIGH
error_tc = digitalRead(SO); // Read data
digitalWrite(SCK,0); // Set Clock to LOW
digitalWrite(TC_0, 1); //Disable Device
last_read_time = millis(); //remember the read time for next time
if(error_tc)
currentTemperature = 2000;
else
currentTemperature = value>>2;
}
#endif
}

The arduino library can be improved by simply inserting the following at line 55 of the file MAX6675.cpp just before the end of the for loop.

if (i>1) delay(220); // Wait 220ms for next sample to be ready

This will force a 220ms delay when reading the thermocouple multiple times via the read_temp(int samples) function. Alternatively you could use the millis() function to track the last time it was read and automatically delay the read by the appropriate amount.